Beyond my ken to approximate it in a CAD drawing, but I like the "minimum energy shape", if it both fulfils the desired hydrodynamic shape, and facilitates easier construction. I think you have established that the original straight-sectioned drawing can't be met, in practice, and it has become what can be done, and in which way, which is some progress. The curved bottom aft is the preferred shape, I think, but not so easy to get the plate to conform, perhaps.

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My guess is the second alternative with the curved bottom aft should be easier to plate over framework with the appropriate curves. I don't have Andrei's (original poster's) surface and don't have experience to say whether it could or could not be built.

I'm curious how the second shape would perform at planing speeds. It almost has a planing pad aft. Either a test with a model or actual boat, or some good (emphasis good) CFD modeling would be needed. Savitsky analysis and similar would not provide more than an approximation.

If you are going to put extra buoyancy aft, by selecting a particular shaping option over another, retaining the existing chine and keel, I'm not sure I like it, it is already a warped plane, as drawn.

If you are going to put extra buoyancy aft, by selecting a particular shaping option over another, retaining the existing chine and keel, I'm not sure I like it, it is already a warped plane, as drawn.

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Not thinking of any modifications. I've been exploring some warped bottom shapes for plywood / cold molding which do not need to be exactly developable.

My guess is the second alternative with the curved bottom aft should be easier to plate over framework with the appropriate curves. ....

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There are, as you have now nicely shown to him, many ways in which this can be done. It is a pity it is after the fact, rather than at the early design stage!

Bottom line is, what skill/experience and tools are available to the plater making the boat?
That is what dictates the shape and thus, the method, that should be used in creating the hull bottom shape. Since no point creating a nice rolled/conical shape, if the tools are not available.

An aft triangular section of the bottom is not uniquely determined by the keel and chine curves and developable surface assumption due to the warped bottom. The designer has to decide how to shape that portion of the bottom. Creating a conical surface with the apex at the aft end of the chine is a simple method to create the remainder of the bottom. The resulting surface has a small radius "kink" running diagonally across the bottom with a planar triangle behind it. The two portions of the bottom will be tangent but the curvature will be discontinuous between them.While this is technically developable it is likely to be difficult to bend plate to the small radius and curvature discontinuity, particularly close to the aft end of the chine where the radius approaches zero. It also is may not be desirable for good water flow and boat performance.View attachment 145780View attachment 145781View attachment 145782View attachment 145783

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Hi David, big thank you for your feedback that I do find admirable. I am reading and studying your above , and of course yes the bottom you created does look beautiful. I think the forum thread we made here is very good because all the above feedbacks and considerations are very useful to the design process and to the making of small aluminium boat series production. All of the above are to be carefully integrated in a positive and balanced manner in order to make sure a series production run is achievable with accuracy and of course that the boats delivered are beautiful and do work well. I am working to further improve this particular development of the design of X580 aluminium motor boat the boat is to be configured in a number of versions and this particular hull was supposed to be more suited to a version with cabin . I will very likely change the concept of making the entire bottom framing as a distinct module .

There are more ways to skin a cat, with a slight rocker to the keel line, and a curved bottom at the transom, you can get a developed shape with a pretty straight vee at the forefoot, with good deadrise.

There are more ways to skin a cat, with a slight rocker to the keel line, and a curved bottom at the transom, you can get a developed shape with a pretty straight vee at the forefoot, with good deadrise.View attachment 145800

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Hi Efficiency thanks for feedback yes I do agree. I did a new bottom version that I did with DevSrf. Though mine is not the most perfect in the sense David was illustrating so brilliantly above; I think my bottom will work for the series production and bend naturally on frames ( wich are slightly curved).

I think that there is often a drive to create a design based on a strict geometrical constraint which is not necessarily conducive to a good boat. In this case, the transverse curvature of the sections are probably not detrimental to the design. Why struggle to make the plate lay flat on the forward frame unless that affects the boat's performance? On the other hand, some cuts in the forward area of the plate could be enough to make it lay flat.

Another way you may wish to understand this, is by analysing what is going on with the plate..and shown i your post here.

If i look at the hull lines, body plan and draw a line perpendicular to the frame line surface, shown in blue, and compare it against an extension of the first frame to the aft frame, you can see there is a high degree of twist.

If you wish to, or attempting to make this from one sheet of plate, a simple analysis of what is occurring in the plate structure can be revealing.

The shear stress from torsion or an angle of twist = angle of twist.thickness.Modulus(E).

If the plate were 4mm thick and you decide to use the max shear stress possible for aluminium the plate may twist through an angle of 15 degree before shearing/yielding occurs.

But we need a factor of safety, so not to put any undue or unnecessary residual stress into the plate from twisting it. So, if we, for argument's sake allow just 20% of yield as our guide, the max angle of twist becomes just 3 degrees.
As you can see simply by inspection, the projected angle from the bow to the stern has an angle greater than 15 degrees...not ideal.

You can try this with a simple plastic ruler, hold each end and give it a slight twist. In addition, you'll note that the "frame lines" are no longer straight.

Thus if the objective is to make this hull bottom from one sheet, this must be taken into account, not to over stretch or twist the plate. And there are various options available for this, as noted above by DC and others.

Hi AdHoc thank you for your feedback. Yes I agree to your above . However, I think in defense of my (not 100% perfectly developable ) initial bottom the making of two distinct scale models in steel plate was in this case validation the aluminium bottom plate will do its shape when mounted. This is the first rough steel model 3mm. I think if 3mm steel plate does bend on a 500 mm scale model that should be quite telling. On the other hand I was thinking: the two software that did unroll the boat bottom they do it based on a mathematical algorythm. If a surface is unrolled by Rhino and as well unrolled by paneling tools that happens because that surface is analysed by the developabilty algorythm of these two software. One may consider that algorythm as a valid criteria. Nonetheless people do generally state that the fact a surface is unrolled by Rhino does not truly validate it as being developable. Fewer such statements are to be found in regards the Paneling Tools unroll command and capability.

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Andrei - Do you have any information about how the Rhino Paneling Tools unroll command, ptUnrollFaces, differs from the other Rhino unroll commands, UnrollSrf and UnrollSrfUV? I cannot find any information on the Rhino forum. I have asked about it; perhaps one of the McNeel staff will answer on Monday. A quick experiment shows ptUnrollFaces will unroll a severely warped surface with a large amount of twist. The results appear to be identical to those from UnrollSrf except that ptUnrollFaces does not report the change in area unlike UnrollSrf. (I'm using Rhino V6.)

Your surface is not developable. A developable surface will not have the twist which Ad Hoc discusses. You need to decide if your current surface is close enough to developable for your purposes. If you think it is then proceed. If you want to redesign the bottom as a developable surface or one which is closer to developable I can help. My previous post showed two versions of developable surfaces with keel and chine curves which are very close to yours.

Hi David, your analysis is most welcome. I did not seek information about how the Rhino Paneling Tools unroll command, ptUnrollFaces, differs from the other Rhino unroll commands; my reasoning came from doing work a couple of months with the Paneling Tools on complex systems of developable surfaces and by just observing that the Paneling Tols appeared to not work on surfaces that Rhino did develop - and less often the PT command was unrolling surfaces that Rhino would not. Yes, I think you are right I think it is worth examining as your above. yes I did already do another bottom .

The problem you have is getting the longitudinals to touch the plating without dramas, and how you do that without getting the sectional frame shapes right, is another matter. I'm surprised this developed shape problem wasn't obvious to all and sundry from the beginning of the thread.

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